Might I suggest Dino Blasters in the Actionsack playlist. Unlimited Jetpacks & Concussion Rifles. Also, doesn't appear to be a single game challenge, so don't worry if you mess up. Just remember to hold the equipment button as you get the kill.

L.D. Pillar - Complete The Pillar of Autumn, without dying, with Mythic, Thunderstorm, Tilt, and Touch Luck on - 5000cR

Doesn't say Legendary, but L.D. = Legendary Deathless. If you can get to the last checkpoint, then all you need to do is save & quit. Other than that, having a second person around to do all the work while you focus on staying alive works well.